network-manager not setting nm as renderer with netplan

[Kiley Hykawy]

network-manager snap version (20/stable): 1.22.10-10
network-manager snap revision: 702

We have only see this on one of our Ubuntu Core 20.04 devices but it looks like /etc/netplan/00-default-nm-renderer.yaml is empty (0 bytes). This results in NetworkManager not managing devices as we expect. This breaks functionality for us that uses NetworkManager DBus to get state so we can set device network status LEDs. We don't do any additional NM configuration and we normally see our eth0 device get managed by network manager without any other action.

ubuntu@nureva:~$ network-manager.nmcli d
DEVICE TYPE STATE CONNECTION
eth0 ethernet unmanaged --
lo loopback unmanaged --

ubuntu@nureva:~$ network-manager.nmcli g
STATE CONNECTIVITY WIFI-HW WIFI WWAN-HW WWAN
disconnected unknown enabled enabled enabled enabled

Attached results of `sudo journalctl -xe |grep network-manager`.

[Kiley Hykawy (kileyhykawy-nureva)]

I cannot confirm but I think the device might have got into this state of an empty `00-default-nm-renderer.yaml` because of a power loss sometime during device setup in first boot of Ubuntu Core. I am thinking that a way to recover from this is instead of Network Manager snap checking if file exists (https://git.launchpad.net/~snappy-hwe-team/snappy-hwe-snaps/+git/network-manager/tree/snap-common/bin/snap-config.sh?h=1.22.10-10#n160) to determine if it should write it out, it could check if it does not exist OR the file is empty. Then it could recover on a subsequent boot.

Could also consider using `_replace_file_if_diff`.